The present invention relates generally to molding and forming processes and more specifically to the continuous forming of compound-curved (doubly-curved, complex) shapes (such as shell structures) without the use of molds or dies.
The prior art for mass-producing identical shapes with complex contours includes a variety of molding, casting, and die stamping processes. Almost all such processes have relied on single-purpose tooling in the form of molds, dies, or masters. The disadvantages of these processes come from their reliance on such dedicated tooling and the costs associated with it. In the design and manufacturing stages, a group of specialists in product design, manufacturing engineering, and die-making must work together, with the time and expense of their collaboration becoming less predictable as the degree of novelty and complexity of a desired shape increases. Additionally, the required tooling is often complicated and requires continuing highly-skilled maintenance. Finally, tooling that goes in and out of service has continuing overhead costs associated with storage, including risk of damage, deterioration, and inventory expenses.
These costs, along with the risk of undertaking a project with unpredictable time and expense factors, can reduce the degree of innovation or variety a manufacturer is willing to incorporate into a product line. At the same time, increasing competitive pressures are driving the markets for many goods, with the difference between success or failure for a product line or an entire company often depending on the company's ability to have a quick response to market changes, or to lead the market through product innovation and variety. The capabilities for producing small quantities of highly differentiated products, often on a very compressed timetable, are quickly becoming an absolute necessity in many fields of manufacture.
There have been various attempts to reduce or eliminate the need for single-purpose tooling. For example, U.S. Pat. No. 4,865,796 (Tamura et al.) discloses a technique for forming elongate members for automobiles. The method includes the steps of extruding the material with constant cross-section, continuously removing materials so as to vary the cross-section in the longitudinal direction, and cutting the materials in predetermined lengths. This has the advantage that there is no need for molds or for combining separately formed parts, but is limited to shapes that can be formed by the removal of material from a constant profile of extrusion. There is no provision for longitudinal curvatures.
U.S. Pat. No. 4,770,017 (Yamashita et al.) discloses a technique for forming plates into double-curved shapes. The technique relies on bending and stretch-deformation induced by passing the plate through an entrance roll, flexible rolls, and exit rolls. The flexible rolls can assume various profiles to give transverse curvature while the three sets of rolls are adjusted vertically to control longitudinal curvatures. Again, this technique has the advantage that there is no need for approximating dies or manual heating and bending to final shape, and it develops smoother surfaces than shapes which are repeatedly punched or hammered. However, multiple passes are required for deeply-drawn complex surfaces, and the flexible rolls require numerous wheels with rim shapes which must be varied to form a wide range of plate contours.
U.S. Pat. No. 4,755,334 (Grimm et al.) discloses a molding apparatus that provides some flexibility and ease of adaptation compared to standard molds or dies. Specifically, belt-mounted mold segments form a mold cavity with opposing mold segments or a continuous facing. Thus, it is possible to mold complex shapes of any length, and the mold segments can be rearranged or substituted to mold various shapes. However the segments have fixed contours, requiring a stock of mold segments for each shape. Moreover, the precise mating of mold segment edges is difficult to achieve if the complex contours of the shape extend beyond a single mold segment.
U.S. Pat. No. 4,749,535 (Matsuda) discloses a technique using constant cross-section extruded thermo-plastic material which is introduced into a contoured molding roll, rapidly cooled, and cut to length. This is a simple apparatus for producing compound-curved shapes with a single forming surface that does not require pressure or vacuum apparatus for thermoforming. However, the formed shapes cannot have longitudinal concavities because extruded material would bridge such concavities in the forming surface, and the rotating roll which bears the forming surface must be scaled to the size of the desired shape, drastically limiting the shape range of a particular molding roll. Thus, the prior art techniques have either yielded to the need for fixed tooling, or have escaped such need at the cost of being unable to form complex shapes.